The industry classification system used during Census 2000 was developed for the census and consists of 265 categories for employed people, classified into 14 major industry groups. From 1940 through 1990, the industrial classification has been based on the Standard Industrial Classification (SIC) Manual . The Census 2000 classification was developed from the 1997 North American Industry Classification System (NAICS) published by the Office of Management and Budget, Executive Office of the President. NAICS is an industry description system that groups establishments into industries based on the activities in which they are primarily engaged.

The NAICS differs from most industry classifications because it is a supply-based, or production-oriented economic concept. Census data, which were collected from households, differ in detail and nature from those obtained from establishment surveys. Therefore, the census classification system, while defined in NAICS terms, cannot reflect the full detail in all categories. NAICS shows a more detailed hierarchical structure than that used for Census 2000. The expansion from 11 divisions in the SIC to 20 sectors in the NAICS provides groupings that are meaningful and useful for economic analysis. Various statistical programs that previously sampled or published at the SIC levels face problems with the coverage for 20 sectors instead of 11 divisions. These programs requested an alternative aggregation structure for production purposes which was approved and issued by the Office of Management and Budget on May 15, 2001, in the clarification Memorandum No. 2, "NAICS Alternate Aggregation Structure for Use by U.S. Statistical Agencies." Several census data products will use the alternative aggregation, while others, such as Summary File 3 and Summary File 4, will use more detail.

The occupational classification system used during Census 2000 consists of 509 specific occupational categories for employed people arranged into 23 major occupational groups. This classification was developed based on the Standard Occupational Classification (SOC) Manual: 2000, which includes a hierarchical structure showing 23 major occupational groups divided into 96 minor groups, 449 broad groups, and 821 detailed occupations. For Census 2000, tabulations with occupation as the primary characteristic present several levels of occupational detail.

Some occupation groups are related closely to certain industries. Operators of transportation equipment, farm operators and workers, and healthcare providers account for major portions of their respective industries of transportation, agriculture, and health care. However, the industry categories include people in other occupations. For example, people employed in agriculture include truck drivers and bookkeepers; people employed in the transportation industry include mechanics, freight handlers, and payroll clerks; and people employed in the health care industry include occupations such as security guard and secretary.

All civilians 16 years old and over who were either (1) "at work" - those who did any work at all during the reference week as paid employees, worked in their own business or profession, worked on their own farm, or worked 15 hours or more as unpaid workers on a family farm or in a family business; or (2) were "with a job but not at work" - those who did not work during the reference week, but who had jobs or businesses from which they were temporarily absent because of illness, bad weather, industrial dispute, vacation, or other personal reasons. Excluded from the employed are people whose only activity consisted of work around their own house (painting, repairing, or own home housework) or unpaid volunteer work for religious, charitable, and similar organizations. Also excluded are all institutionalized people and people on active duty in the United States Armed Forces.